基础数据结构

背包

背包是一种不支持从中删除元素的集合数据类型--它的目的就是帮助用例收集元素并迭代遍历所有收集到的元素。 迭代的顺序不确定并且与用例无关。

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Bag<Item> implements Iterable<Item> {
    private Node<Item> first;    // beginning of bag
    private int n;               // number of elements in bag

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }

    public Bag() {
        first = null;
        n = 0;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public int size() {
        return n;
    }

    public void add(Item item) {
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        n++;
    }

    public Iterator<Item> iterator()  {
        return new LinkedIterator(first);  
    }

    // an iterator, doesn't implement remove() since it's optional
    private class LinkedIterator implements Iterator<Item> {
        private Node<Item> current;

        public LinkedIterator(Node<Item> first) {
            current = first;
        }

        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }
}

队列

队列是一种基于先进先出（FIFO）策略的集合类型。

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Queue<Item> implements Iterable<Item> {
    private Node<Item> first;    // beginning of queue
    private Node<Item> last;     // end of queue
    private int n;               // number of elements on queue

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }

    public Queue() {
        first = null;
        last  = null;
        n = 0;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public int size() {
        return n;
    }

    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        return first.item;
    }

    public void enqueue(Item item) {
        Node<Item> oldlast = last;
        last = new Node<Item>();
        last.item = item;
        last.next = null;
        if (isEmpty()) first = last;
        else           oldlast.next = last;
        n++;
    }

    public Item dequeue() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        Item item = first.item;
        first = first.next;
        n--;
        if (isEmpty()) last = null;   // to avoid loitering
        return item;
    }

    public String toString() {
        StringBuilder s = new StringBuilder();
        for (Item item : this) {
            s.append(item);
            s.append(' ');
        }
        return s.toString();
    } 

    public Iterator<Item> iterator()  {
        return new LinkedIterator(first);  
    }

    // an iterator, doesn't implement remove() since it's optional
    private class LinkedIterator implements Iterator<Item> {
        private Node<Item> current;

        public LinkedIterator(Node<Item> first) {
            current = first;
        }

        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }
}

栈

栈是一种基于后进先出（LIFO）策略的集合类型。

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Stack<Item> implements Iterable<Item> {
    private Node<Item> first;     // top of stack
    private int n;                // size of the stack

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }

    public Stack() {
        first = null;
        n = 0;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public int size() {
        return n;
    }

    public void push(Item item) {
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        n++;
    }

    public Item pop() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        Item item = first.item;        // save item to return
        first = first.next;            // delete first node
        n--;
        return item;                   // return the saved item
    }


    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        return first.item;
    }

    public String toString() {
        StringBuilder s = new StringBuilder();
        for (Item item : this) {
            s.append(item);
            s.append(' ');
        }
        return s.toString();
    }
       
    public Iterator<Item> iterator() {
        return new LinkedIterator(first);
    }

    // an iterator, doesn't implement remove() since it's optional
    private class LinkedIterator implements Iterator<Item> {
        private Node<Item> current;

        public LinkedIterator(Node<Item> first) {
            current = first;
        }

        public boolean hasNext() {
            return current != null;
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }
}

Dijkstra的双栈算术表达式求值算法

表达式由括号、运算符和操作数（数字）组成。我们根据以下4种情况从左到右逐个将这些实体送入栈处理：

• 将操作数压入操作数栈
• 将运算符压入运算符栈
• 忽略左括号
• 在遇到右括号时，弹出一个运算符，弹出所需数量的操作数，并将运算符和操作数的运算结果压入操作数栈

public class Evaluate {
    public static void main(String[] args) { 
        Stack<String> ops  = new Stack<String>();
        Stack<Double> vals = new Stack<Double>();

        while (!StdIn.isEmpty()) {
            String s = StdIn.readString();
            if      (s.equals("("))               ;
            else if (s.equals("+"))    ops.push(s);
            else if (s.equals("-"))    ops.push(s);
            else if (s.equals("*"))    ops.push(s);
            else if (s.equals("/"))    ops.push(s);
            else if (s.equals("sqrt")) ops.push(s);
            else if (s.equals(")")) {
                String op = ops.pop();
                double v = vals.pop();
                if      (op.equals("+"))    v = vals.pop() + v;
                else if (op.equals("-"))    v = vals.pop() - v;
                else if (op.equals("*"))    v = vals.pop() * v;
                else if (op.equals("/"))    v = vals.pop() / v;
                else if (op.equals("sqrt")) v = Math.sqrt(v);
                vals.push(v);
            }
            else vals.push(Double.parseDouble(s));
        }
        StdOut.println(vals.pop());
    }
}